A known method for epitaxial growing of an AlN monocrystal comprises supplying vapors of Al and N2 to a growth chamber, with the pressure of N2 being above the stoichiometric pressure and the total pressure in the chamber being above the atmospheric pressure. At least one nucleation center is placed in the chamber, and said center is cooled down relative to the other contents of the growth chamber. Then the vapor of Al and N2 is supplied to the chamber for growing AlN at the nucleation site, see U.S. Pat. No. 6,770,135 B2.
The main disadvantage of said method consists in that the source material for growing an AlN monocrystal is AlN powder that requires extremely high temperature (2000-2500° C.) for decomposition thereof, which drastically increases the equipment costs, because expensive very high-temperature and hard-to-process construction materials have to be used in addition, using the source material in the form of a powder results in it being oxidized by oxygen in the air prior to the start of process, which increases the number of flaws in the obtained product and results in insufficient transparency.
Another known method for epitaxial growing of monocrystalline aluminum nitride from a mixture of aluminum vapors and nitrogen comprises arranging a substrate and the Al source opposite one another in a growth chamber, heating and maintaining working temperatures of said source and said substrate that ensure, respectively, the production of aluminum vapors within the mixture and the growth of aluminum nitride monocrystal on the substrate; the pressure of the mixture of nitrogen and aluminum vapor in the growth chamber is maintained within 400 mbar from the lower value that equals the pressure created in a closed space by a stoichiometric mixture of nitrogen and aluminum vapors that is produced through evaporation of the source material, said mixture having the 1:1 ratio of concentrations of nitrogen and aluminum atoms, see RU 2158789 C1.
The disadvantage of said method consists in the utilization of molecular nitrogen, the temperature of decomposition of which into atoms on a hot surface equals 2000-2500° C.; in order to allow carrying out this process at such high temperatures, a material is used that contacts the growth zone, said material being embodied as a solid solution of tantalum carbide in tantalum; at high temperatures the carbon contained the tantalum carbide enters the content of the obtained product that leads to the emergence of flaws in the AlN monocrystal.
The abovementioned methods relate to methods for epitaxial growing of an AlN monocrystal through sublimation of the source materials. As specified above, these methods require very high temperatures for the implementation thereof.
There are known methods for epitaxial growing of an AlN monocrystal by gas-phase epitaxy.
RU 2158789 C1 describes a method for epitaxial growing of an AlN monocrystal by gas-phase epitaxy, in which the nitrogen source is embodied as molecular nitrogen, and the source of aluminum is embodied as free Al. The disadvantage of said method, along with all other methods in which molecular nitrogen is used, consists in the fact that said methods are implemented at quite high temperatures in the growth zone (in practice, at not lower than 2000° C.), which requires special equipment made of high-temperature materials that are treated using specialized complex technologies. In particular, parts made of tungsten and its alloys that are used in such equipment are produced using quite expensive methods of powder metallurgy.
In certain other known methods for epitaxial growing of an AlN monocrystal by gas-phase epitaxy the nitrogen source is implemented not as molecular nitrogen but as a compound thereof—ammonia (NH3), and the Al source is embodied as a mixture of free Al and AlN. These methods are implemented at lower temperatures.
In particular, in a known method for epitaxial growing of a monocrystal of nitrides of metals of 3A group of chemical elements, including a monocrystal of AlN, the Al source is embodied as a mixture containing free Al and a nitride component—AlN, and the nitrogen source is embodied as NH3. The Al source and a substrate, with the growth surface of said substrate turned towards said Al source, are arranged opposite one another in a growth chamber, said source and substrate forming a growth zone. A flow of NH3 is supplied to the growth zone on the side through a porous crucible and between the grains of the mixture of Al and AlN, and the mixture of Al and AlN and the substrate are heated to temperatures that ensure the growth of the monocrystal on the substrate, see RU 2097452 C1.
This technical solution has been taken as a prototype of the method of the present invention.
In this method, as in other known methods, the Al source can only be embodied as a mixture of free Al and AlN. If only free (metallic) Al is used in the prototype method then the passing of NH3 through the metallic aluminum melt causes the formation of a layer of solid AlN on the melt surface, which blocks the evaporation of Al at temperatures that correspond to evaporation of metallic aluminum. In order to evaporate said layer of AlN, the temperature would have to be increased to ≧1800° C., which would effectively nullify all advantages of the method that uses NH3. Therefore in the known methods that use NH3 as the nitrogen source, free Al has not been used as the Al source, and the Al source has been embodied as a mixture of Al and AlN.
However, combined utilization of free Al and AlN as an Al source is only possible in the form of a powder mixture of AlN and free (metallic) Al. Said powder has quite large total surface area of particles, which results in intensive formation of oxides due to a reaction with the oxygen in the air over a large surface, contamination of the obtained material—AlN monocrystal and, correspondingly, causes high extent of flaws thereof, up to the point where the obtained material is not monocrystalline but polycrystalline. Besides, using a mixture of free Al and AlN as the Al source leads to other negative consequences as well. AlN in a vacuum growth chamber evaporates at t°≧1800° C. (see http://en.wikipedia.org/wiki/Aluminium_nitride), whereas Al starts to evaporate at t°=1050° C. During the heating of the Al and AlN mixture to t°≧1800° C. the metallic Al will melt and will start evaporating and escaping much sooner that the evaporation of AlN will start, therefore in the prototype method the growth of AlN monocrystal will take place during all stages at quite high growth rate. That said, at the initial stage of growing of an AlN monocrystal it is very important to ensure that the number of flaws in the first layers of the monocrystal is minimal, wherein said number of flaws increases upon increasing the growth rate of the AlN monocrystal, which is directly proportional to the temperature of Al source.
In addition, the necessity of heating the source mixture of Al and AlN to high; temperatures requires using quite expensive construction materials for building the growth chamber and the elements arranged therein. The fact that the prototype method does not comprise pretreating the growth surface of the substrate also increases the number of flaws in the AlN monocrystal and may result in an unacceptable discoloration thereof, to the extent that it may become amber brown. This makes the AlN monocrystal unsuitable, in particular, for using in the ultraviolet range.
A known device for implementing the method for growing an AlN monocrystal by gas-phase epitaxy comprises a body, a growth zone heater, a substrate holder, a crucible for arranging the Al source in the form of a cylindrical chamber and an Al source heater, see RU 2158789 C1.
This technical solution has been taken as a prototype of the device of the present invention.
Its disadvantage, same as for the device used in RU 2097452 C1, consists in the fact that NH3 comes in directly contact with the Al source, which does not allow using free Al as the Al source due to the formation of a layer of solid AlN on the Al melt surface upon its contact with NH3, which blocks the evaporation of Al.